1. Field of the Invention
The invention deals with the field of creating print masters, and more specifically with digital methods and systems for creating a digital flexographic print master on a drum by a fluid depositing printhead.
The invention reduces a problem that may result when a printhead unit is used that comprises more than one nozzle row.
2. Description of the Related Art
In flexographic printing or flexography a flexible cylindrical relief print master is used for transferring a fast drying ink from an anilox roller to a printable substrate. The print master can be a flexible plate that is mounted on a cylinder, or it can be a cylindrical sleeve.
The raised portions of the relief print master define the image features that are to be printed.
Because the flexographic print master has elastic properties, the process is particularly suitable for printing on a wide range of printable substrates including for example, corrugated fiberboard, plastic films, or even metal sheets.
A traditional method for creating a print master uses a light sensitive polymerisable sheet that is exposed by a UV radiation source through a negative film or a negative mask layer (“LAMS”-system) that defines the image features. Under the influence of the UV radiation, the sheet will polymerize underneath the transparent portions of the film. The remaining portions are removed, and what remains is a positive relief printing plate.
In the unpublished applications EP08172281.1 and EP08172280.3, both assigned to Agfa Graphics NV and having a priority date of 2008-12-19, a digital solution is presented for creating a relief print master using a fluid droplet depositing printhead.
The application EP08172280.3 teaches that a relief print master can be digitally represented by a stack of two-dimensional layers and discloses a method for calculating these two-dimensional layers.
The application EP08172281.1 teaches a method for spatially diffusing nozzle related artifacts in the three dimensions of the stack of two-dimensional layers.
Both applications also teach a composition of a fluid that can be used for printing a relief print master, and a method and apparatus for printing such a relief print master.
FIG. 1 shows an embodiment of such an apparatus 100. 140 is a rotating drum that is driven by a motor 110. A printhead 160 moves in a slow scan direction Y parallel with the axis of the drum at a linear velocity that is coupled to the rotational speed X of the drum. The printhead jets droplets of a polymerisable fluid onto a removable sleeve 130 that is mounted on the drum 140. These droplets are gradually cured by a curing source 150 that moves along with the printhead and provides local curing. When the relief print master 130 has been printed, the curing source 170 provides an optional and final curing step that determines the final physical characteristics of the relief print master 120.
An example of a printhead is shown in FIG. 3. The printhead 300 has nozzles 310 that are arranged on a single axis 320 and that have a periodic nozzle pitch 330. The orifices of the nozzles are located in a nozzle plate that is substantially planar.
FIG. 2 demonstrates that, as the printhead moves from left to right in the direction Y, droplets 250 are jetted onto the sleeve 240, whereby the “leading” part 211 of the printhead 210 prints droplets that belong to a lower layer 220, whereas the “trailing” part 212 of the printhead 210 prints droplets of an upper layer 230.
Because in the apparatus in FIGS. 1 and 2 the linear velocity of the printhead in the direction Y is locked with the rotational speed X of the cylindrical sleeve 130, 240, each nozzle of the printhead jets fluid along a spiral path on the rotating drum. This is illustrated in FIG. 5, where it is shown that fluid droplets ejected by nozzle 1 describe a spiral path 520 that has a pitch 510.
In FIG. 5, the pitch 510 of the spiral path 520 was selected to be exactly double the length of the nozzle pitch 530 of the printhead 540. The effect of this is that all the droplets of nozzles 1, 3, 5 having an odd index number fall on the first spiral path 520, whereas the droplets ejected by nozzles 2, 4, 6 having an even index number fall on the second spiral path 550. Both spiral paths 520, 550 are interlaced and spaced at an even distance 560 that corresponds with the nozzle pitch 530.
The lowest value of the nozzle pitch 330 in FIG. 3 is constrained by technical limitations in the production of a printhead. One solution to overcome this constraint is to use a multiple printhead unit.
The concept of a multiple printhead unit is explained by means of FIG. 4. As the figure shows, two printheads 401 and 402 are mounted to form a multiple printhead unit 400. The nozzle rows 420 and 421 are substantially parallel. By staggering the position of the nozzles 410 on the axis 420 of head 401 and the nozzles 411 on axis 421 of printhead 402 over a distance of half a nozzle pitch, the effective nozzle pitch 431 of the multiple printhead unit is half the nozzle pitch of each constituting printhead 401, 402 and the effective printing resolution is doubled.
The use of a multiple printhead unit in an apparatus as shown in FIG. 1 or FIG. 2 for the purpose of printing a relief print master introduces an unexpected and undesirable side effect.
FIG. 6. shows a first spiral path 610 on which fluid droplets from the nozzles having an odd index number 1, 3 and 5 land and a second spiral path 611 on which the fluid droplets of the nozzles having an even index number 2, 4 and 6 land.
The nozzles with an odd index number are located on a first axis 620 and the nozzles having an even index number are located on a second axis 621, parallel with the first axis 620.
Because these two axes 620 and 621 of the nozzle rows in the multiple printhead unit are not congruent, the spiral paths 610 and 611 are not evenly spaced with regard to each other. For example, in FIG. 6 the distance 640 is different from the distance 641.
The uneven spacing of the spiral paths 610 and 611 causes an uneven distribution of the fluid droplets along the Y direction when they are jetted onto the sleeve and this negatively affects the quality of the print master that is printed.